Governments have become increasingly concerned with and have increasingly regulated the emissions of VOC's such as organic paint and adhesive solvents and organic fabric dry cleaning solvents. Typical organic paint solvents include MEK, xylene, toluene, isopropyl alcohol and lacquer thinner. Two methods have been used in the past to abate the discharge into the atmosphere of VOC emissions from paint spray booths. According to one method, at least a portion of the air circulated through a paint spray booth is withdrawn and passed through a burner which incinerates the VOC's. Direct incineration of the VOC's can consume a large amount of energy, since the exhausted air must be heated to approximately 1500.degree. F. to 1700.degree. F. (815.degree. C. to 925.degree. C.) during the entire time that the VOC's are emitted. Further, in colder climates it may be necessary to heat a large amount of make up air which must be supplied to the spray booth to replace the exhausted air.
A second VOC abatement method involves collection of the VOC's in a filter bed, such as a charcoal filter bed. However, the filter bed either must be frequently replaced and properly disposed of or must be periodically regenerated by desorbing the accumulated VOC's. A charcoal filter bed requires about 600.degree. F. (about 315.degree. C.) for regeneration. When a charcoal filter bed is used for collecting VOC's, there is a risk that an exothermic reaction can occur, creating sufficient heat to cause combustion. It also is known that certain adsorbent materials such as zeolite have a high molecular affinity for VOC's and other vapors and gases, and may be used as a filter material for separating components of a gaseous mixture. A zeolite filter typically requires heating to about 300.degree. F. to 350.degree. F. (about 150.degree. C. to 175.degree. C.), or more, for regeneration. Because of the intense heat required for regeneration, it was necessary to transfer a carbon filter bed or a zeolite filter bed from the adsorption location in a spray booth to a separate furnace for regeneration. Separate heat sources were required for heating the paint in the spray booth to cure the applied paint and for regenerating the filter.
It also is known that VOC's can be eliminated from a gas stream by oxidation without reaching the combustion temperature. The VOC vapor in a gas stream can be oxidized by heating and contacting with a catalyst such as platinum. Typically, the catalyst is coated on a porous substrate, such as a ceramic honeycomb or foam, which provides a very large surface area for contact with the gas. Both catalytic oxidation and incineration are effective to reduce the emitted VOC's to carbon dioxide and water before the exhaust gas is released to the atmosphere.
Spray booths are generally used, for example, in commercial shops which repaint vehicles, in order to confine paint overspray and to evacuate VOC's released during painting. Generally, the spray booth air has been exhausted to the atmosphere. Filtered spray booth air has been recirculated back to the booth only when the booth is unmanned. Preferably, a high flow of air is passed through the spray booth filtered during painting in order to maintain clean filtered air inside the spray booth to reduce the risk of paint overspray, dust and environmental contaminants from outside the booth from contacting and damaging the wet finish. The exhausted booth air is easily filtered to remove particulate overspray. However, the highest level of VOC's are released during and immediately after the paint spraying operation. The booth air could not be recirculated during spraying in a manned booth since the painter would be subjected to an increasing VOC vapor level in the booth as painting progresses. If the spray booth air is exhausted to the atmosphere, a large quantity of clean, warm make up air is required. In colder climates, the make up air often needs to be heated. Preferably, the spray booth air is maintained at about 70.degree. F. (21.degree. C.) during painting and the booth temperature may be increased significantly during the drying and baking stage. For example, the painted surfaces and the booth air may be heated to reach about 140.degree. F. to 180.degree. F. (60.degree. C. to 82.degree. C.) during drying. A paint spray booth of a sufficient size for enclosing a vehicle may have an air flow rate on the order of 10,000 cubic feet per minute (283 cubic meters per minute). To exhaust this amount of air from the booth and replace it with clean, warm make up air can consume a large amount of energy in addition to the energy required for removing the VOC's from the exhausted air. This adds significantly to the cost of painting a vehicle. Ideally, any particulates such as paint overspray and the VOC's is filtered or otherwise separated from the warm spray booth air and the warm filtered air is then returned to the booth.